Asthma is a chronic airway inflammatory disorder characterized by bronchial hyper-reactivity and bronchospasm, among other abnormalities. Lungs of asthmatic patients have increased numbers of inflammatory cells in bronchioalveolar fluid and in lung tissues. These inflammatory cells include eosinophils, basophils, neutrophils, macrophages, and lymphocytes. In asthmatic lungs, the epithelium, including ciliated columnar epithelial cells, is damaged. IgE-antigen-mast cell interactions represent the early molecular and cellular events that cause inflammatory conditions of asthma.
Mast Cell Stabilizing Agents provide one approach to the prophylaxis and/or treatment of asthma. The prototype drug, disodium cromoglycate was synthesized in 1965 and was approved in the United States in 1973 as a prophylactic, nonbronchodilating anti-inflammatory drug for the therapy of allergic disorder. Cromolyn is an odorless, white, hygroscopic crystalline powder that is freely soluble in water up to 5%. Animal and human studies show it to be excreted unchanged in bile and urine. When inhaled into the pulmonary tree, as for treatment of asthma, only about 8% of a dose is deposited in the lung and absorbed. Peak plasma levels occur within 15 minutes, the biologic half-life is 46-99 minutes. Oral administration in humans results in approximately 1% being systemically absorbed. Cromolyn toxicity studies show an impressively low order of acute toxicity, and adverse effects tend to be minimal and reversible. Cromolyn has a unique, purely prophylactic action with no intrinsic bronchodilator or antihistaminic activity. Nedocromil was introduced subsequent to cromolyn. It is the other standard mast cell stabilizer used in the treatment of asthma. Its chemical properties and therapeutic characteristics are similar.
Nitric oxide (NO) is a diffusible radical involved in many physiological and pathological processes. It is synthesized in vivo by oxidation of L-arginine. The synthesis is catalyzed by a family of enzymes known as nitric oxide synthases or NO-synthases (NOSs), which are referenced in the international enzyme nomenclature system under the number E.C.1.14.13.39. Three NOS isoforms, two of which are constitutive and one inducible, are known:
(1) A neuronal NOS(NOS-1 or nNOS) was originally isolated and cloned from nerve tissue in which it is a constitutive enzyme. nNOS produces NO in response to various physiological stimuli, such as the activation of membrane receptors, according to a mechanism dependent on calcium and on calmodulin. nNOS-derived NO serves as a neurotransmitter.(2) An inducible NOS(NOS-2 or iNOS) can be induced in response to immunological stimuli such as, for example, cytokines or bacterial antigens in various cells such as, for example macrophages, epithelial cells, hepatocytes, glial cells, and other cell types. The activity of this isoform is not regulated by calcium. Once induced, it produces large amounts of NO over prolonged periods.(3) An endothelial NOS(NOS-3 or eNOS) is constitutive and calcium/calmodulin-dependent. It was originally identified in vascular endothelial cells, in which it generates NO in response to physiological stimuli such as the activation of membrane receptors.
Nitric oxide produced by eNOS and nNOS plays a critical role in cellular signaling and acts to control numerous physiologic functions including vasodilation and bronchodilation in the lung. In the asthmatic lung, eNOS and nNOS are downregulated, and thus contribute to edema and bronchoconstriction. Contemplating the problem of inadequate eNOS and nNOS activity, in an approach which is the opposite of that taken in the present invention, Garvey et al. (US published application 2003/0199529) have attached stimulators of endogenous NO production to mast cell inhibitors.
The NO produced in large amounts by the inducible isoform iNOS is involved in pathological phenomena associated with acute and chronic inflammatory processes in a large variety of tissues and organs. NO is highly reactive and, together with superoxide, forms peroxynitrite which damages tissues. In asthma this results in epithelial cell extrusion, sloughing, and cessation of cilia function. An excessive production of NO by induction of iNOS thus plays a part in degenerative pathologies with inflammatory components, such as asthma.
In conditions in which an overproduction of NO is deleterious, it would be desirable to reduce the production of NO by administering substances capable of inhibiting iNOS. However, given the important physiological roles played by the constitutive isoforms, selective inhibition of iNOS is required.